A patient has been identified whose plasma mediates breakdown (presumably proteolysis) of normal von Willebrand factor (vWF). Proteolysis of vWF nor-mally occurs through the action of a plasma enzyme that has recently been characterized; it accounts for the small quantities of cleavage products normally present in the circulation. Other enzymes such as plasmin and leukocyte elastase also can cleave vWF. Preliminary studies with this patients plasma have shown that there appear to be differences in cation and pH requirements of the patients enzyme when compared with the previously charac-terized enzyme; further purification is being carried out to better characterize the activity. We have developed a better and more rapid assay to evaluate the cleavage of vWF multi-mers and have characterized the dose response (rough quantitation of the enzyme in the patients plasma).